With Latest Climate Commitments, How Much Will the World Warm? It's Complicated.

Even with new climate action commitments, studies show the world is expected to warm by 2.7-3.7°C over the next century. Photo by Black Rock Solar/Flickr

Earlier this month, the UNEP Emissions Gap Report joined a series of studies released over the past few weeks assessing how much countries’ recent climate action commitments, or intended nationally determined contributions (INDCs), will combat warming. Collectively, the studies make it clear that the INDCs will substantially bend the global emissions trajectory below our current path, but still don’t go far enough to limit warming to below 2°C and avoid some of the worst climate impacts.

But determining the world’s exact future emissions trajectory or how much global temperatures will increase is a bit more complicated. There are more than a dozen scenarios assessing countries’ INDCs, all with slightly different assumptions and findings.

Here, we analyze the latest studies in order to answer common questions about the INDCs and their potential impact on emissions and global temperature rise. (Also see our more in-depth coverage of this topic.)

What do the latest studies examine?

The studies compare global emissions pathways resulting from the INDCs to one or more alternative scenarios without the INDCs. They find that the INDCs reduce future emissions relative to these scenarios. Some studies also look at the impact of expected emissions on temperature.

What do they tell us about how much the world is expected to warm?

The studies find that the INDCs put us on track for a world that warms by 2.7-3.7°C (median chance) over pre-industrial levels over the next century, depending on modeling assumptions. This is an improvement over previous trends, which would lead to 4-5°Cof warming, but falls short of the global goal to limit warming to below 2°C (3.6°F).

Why is it important that warming is limited to 2°C?

There is no “safe” level of warming for all ecosystems and communities around the world. Even with warming of 2°C, we can expect to see adverse effects on water availability in critical river basins, a more than doubling of forest fires in Amazonia by 2050, impacted coral reef recovery from bleaching events, among other effects. However, as warming exceeds 2°C , we will see even more significant impacts.

Every degree matters. For example, with each degree of warming, renewable water resources are projected to decline by at least 20 percent for an additional 7 percent of the global population. With warming greater than 2°C, there is a high risk of abrupt and irreversible changes to ecosystems such as forests, which would lead to “substantial additional climate change” considering that trees sequester significant amounts of carbon dioxide. And in a 4°C-warmer world, widespread coral reef mortality and food shortages are expected.

The warming avoided by the INDCs helps avoid some of these impacts, but not the most dangerous ones. More action is required.

What do the studies tell us about future emissions?

All studies included in our analysis find that emissions levels in 2025 and 2030 are higher than those consistent with a likely chance of limiting warming to 2°C . The UNEP Emissions Gap Report finds that for a least-cost emissions pathway consistent with a likely chance of limiting warming to 2°C, emissions need to be 48 Gt CO2e in 2025 and 42 Gt CO2e in 2030. The emissions levels in the studies range from 51.1-57.2 Gt CO2e in 2025 and 52-61.1 Gt CO2e in 2030. The chart below shows median values for various studies’ assessments of anticipated emissions levels in 2025 and 2030, given the INDCs.

What’s the relationship between emissions in 2025 and 2030 and warming over the next century?

Studies take into account the INDCs to estimate what emissions will be in 2025 and 2030. Emissions levels in these years have significant consequences for our ability to limit warming to 2°C. The higher emissions are in the near term, the greater the required emissions reductions in later decades for limiting warming. Steep rates of emissions reductions are far costlier than more gradual rates of decline. They also risk failing to achieve the 2°C target, and rely more on carbon dioxide removal technologies (e.g. bioenergy and carbon capture and storage), which have yet to be proven at scale.

Why are these studies reaching different estimates?

It’s complicated to take more than 150 INDCs and compare them because they are so diverse in form and content. Each organization approached this task slightly differently. For some countries, estimating future emissions is straightforward, and results in relatively little difference across studies. For others, analysts have to make more assumptions. Either way, the emissions covered by INDCs are then aggregated with projected future emissions from countries, sectors and gases not covered by INDCs. The latter are taken from projections of what future emissions will be under “business as usual” or under current policies.

Three factors are largely responsible for the differences among studies:

Which INDCs the analysis covers: Studies completed earlier generally do not include as many INDCs as later studies, because many countries only recently released their INDCs. In addition, some studies evaluate only those INDC elements that countries have pledged to achieve unconditionally, whereas other studies look also at INDCs that are dependent on receiving international financing or other forms of support (known as a “conditional” INDC).

Assumptions about the INDCs: Not all INDCs are clear about the emissions that will result from their implementation, so studies make different assumptions about this. In addition, studies rely on different data sources both for historical emissions as well as projected emissions. Finally, the way in which emissions reductions and enhanced carbon sequestration are accounted for can have an impact on estimates of future emissions.

Assumptions about what happens after the INDCs: Most INDCs target 2025 or 2030 as the year they will achieve their goals. So one of the most significant factors for future emissions projections is what the studies assume after these target years. For instance, do actions continue or end after the INDC is completed? Since most INDCs do not specify action after 2030, different scenarios reflect a broader range of assumptions, resulting in divergence in conclusions regarding post-2030 global emissions.

Relatedly, the agreement must also include strong provisions on capacity building, finance and technology transfer to enhance the likelihood of full INDC implementation.

The science tells us where we’re at. COP 21 offers the opportunity to go further. When negotiators come together next month, they’ll need to be prepared to listen to the science and start closing the emissions gap.